The present invention relates to the fields of downhole oil, gas and/or geothermal exploration and more particularly to the fields of tractor systems used to move downhole tools along a borehole with precision movements. Tractor systems of this type are particularly useful in more horizontal borehole sections where weight-on-bit is not as available to move downhole tools. Tractor systems can also be useful when exact advancements are required to obtain accurate readings from downhole sensors.
There currently exists a variety of tractor systems used to move downhole tools. Most downhole tractor systems can be classified in one of two groups: powered-wheel systems and crawler systems. Powered-wheel systems use a plurality of wheels which engage a borehole wall to drive a drill string. Crawler systems use a plurality of arms which extend from the drill string to engage a borehole wall. The arms rhythmically engage and disengage to drive the drill string. The following prior art references disclose various types of crawler systems.
One such crawler system is disclosed in U.S. Pat. No. 6,089,323 to Newman et al., which is herein incorporated by reference for all that it contains. Newman et al. discloses a wellbore tractor system which has at least one slip unit with retractable slips for engaging an interior wall of casing or of a wellbore and at least one movement unit for moving an item. In one aspect while the slip unit is involved in engaging and disengaging from a wellbore, the movement unit moves the item.
Another such crawler system is disclosed in U.S. Pat. No. 6,003,606 to Moore et al., which is herein incorporated by reference for all that it contains. Moore et al. discloses a method of propelling a tool having a generally cylindrical body within a passage using first and second engagement bladders. The first engagement bladder is inflated to assume a position that engages an inner surface of the passage and limits relative movement of the first engagement bladder relative to the inner surface of the passage. An element of the tool then moves with respect to the first engagement bladder. The second engagement bladder is in a position allowing free relative movement between the second engagement bladder and the inner surface of the passage. The first engagement bladder then deflates, allowing free relative movement between the first engagement bladder and the inner surface of the passage. The second engagement bladder is then inflated to assume a position that engages an inner surface of the passage and limits relative movement of the second engagement bladder relative to the inner surface. At this time an element of the tool is moved with respect to the second engagement bladder. This process can be cyclically repeated to allow the tool to generally continuously move forward within the passage.